<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        #app {
            color: red;
        }
    </style>
</head>

<body>
    <div id="app">one</div>
    <div class="one">one1</div>
    <div class="one">one2</div>
    <div class="one">one3</div>
    <script>
        var app = document.getElementById("app");
        console.log(app)
        var one = document.getElementsByClassName("one");
        console.log(one);
    </script>
</body>

</html>